There will always be situations when you will need to write a good cover letter. For example, you may currently hold a staff role but are applying for a supervisory one – use a cover letter to explain why you are qualified for the more senior position. Or, you can explain any gaps in your résumé due circumstances such as travel, school, or motherhood. A cover letter also gives you the opportunity to say something personal about yourself, which can differentiate you from other applicants. Ultimately, the cover letter should make the recruiter's job easier.
Finally, make sure that your cover letter (like your résumé) contains no typos or grammatical errors. Such errors look unprofessional and can detract from the content!